For displaying an image on a digital display, pixels are individually modulated to be either in an `on` condition or in an `off` condition. For example, the `on` condition can represent white and the `off` condition can represent black. To provide more information and realism in the image, it is generally desired to provide intermediate intensities or greyscale levels. Pulse width modulation (PWM) is a well known technique for achieving intermediate greyscale levels. According to PWM, the pixels are rapidly toggled a varying portion of the time between `on` and `off`. The larger a percentage of time the pixel is `on`. the closer to white the pixel is displayed. A viewer's eye integrates the intensity of a toggled pixel over time to perceive grey rather than merely black or white. Thus, the intensity level for the pixel depends upon the relative durations of the `on` state and the `off` state.
To display a complete image, each pixel of the display is modulated according to data representative of the image to be displayed. The spacial resolution of the display image is related to the total number of pixels included in the display; as the number of pixels is increased, the spacial resolution is also increased. U.S. patent application Ser. No. 08/893,872, entitled, "A Dispersion-Based Technique for Modulating Pixels of a Digital Display," filed on the same day as this application and having the same inventor, the contents of which are hereby incorporated by reference, discloses a technique for pulse-width modulating pixels of a display according to an image to be displayed.
It is well understood that pulse-width modulation can also be applied to color systems for forming varying intensities and shades of color. Color display panels differ from black and white display panels in that color display panels generally include a group of three sub-pixels for each pixel, each sub-pixel corresponding to one of the three primary colors, red, green and blue. Typically, a filter of the appropriate primary color is placed over each of the sub-pixels. To display an image, each of the sub-pixels is individually modulated to provide a selected intensity for each of the three primary colors. A viewer's eye perceives a color that is an integration of the three sub-pixel intensities to visualize the selected color for the pixel.
The number of color levels that can be represented in the display by individual pixels (color resolution) is generally limited by the number of bits the display system is capable of accepting and displaying for each sub-pixel. For example, if eight bits are utilized to represent each sub-pixel in a display memory, up to 256 levels of the corresponding primary color can be represented in the display memory for individual sub-pixels. If, however, the display system is capable of accepting only six bits of display data for each sub-pixel, then only sixty-four levels of the corresponding primary color can be displayed by individual sub-pixels on the display panel.
When an image to be displayed includes an area of a color that is not one of the available colors that can be displayed by a pixel, a next closest color can be selected for each pixel in the area. Because the number of available colors is limited, however, the display image will generally not have an appearance that is as realistic or pleasing as is desired. For example, when an image having an area of gradual color change is displayed, the color change will be displayed step-wise, rather than gradually, resulting in an undesired appearance of contouring at the boundaries between steps.
Therefore, it is generally preferred that a digital display have a high color resolution as this generally results in a more pleasing and realistic display image. For example, higher color resolution reduces the tendency for contouring to appear in the display image. As the color resolution is increased, however, so does the number of bits required to represent each sub-pixel. Thus, as the color resolution is increased, the display will generally have a higher cost.
Dithering is a technique for increasing the number of apparent display colors relative to the number of bits actually utilized to represent the intensity for each sub-pixel in the display. Dithering relies upon the viewer's eye integrating colors to visualize a combined color. For example, if it is desired to display an area having a color that is not one of the available colors for display by a pixel, rather than selecting a next closest color for the entire area, pixels of different colors are interspersed with each other in the area The eye integrates the different colors to visualize an intermediate color that is closer to the desired color than any of the colors available for display by individual pixels.
Dithering involves selecting which pixels of the area are to display each of the different colors. According to conventional dithering techniques, the pixels of different colors are selected according to a regular, repeating pattern in the area. A drawback to this technique is that the viewer's eye can often perceive this pattern as artifacts in the image. For example, false motion artifacts, such as lines, can appear to move in the display image. Thus, the expected benefits of dithering, such as reduced contouring in areas of gradual color change and generally more pleasing and realistic display image, can be negated by artifacts introduced as a result of the dithering technique.
Therefore, what is needed is a technique for dithering a digital display system with image data that does not introduce artifacts into the display image.